Conventional rods for rock drilling either have a thread machined directly in the rod, or a thread machined in a rod end forged to a diameter bigger than the rod diameter. Instead of forging up the dimension of rods it is possible to attach, by friction welding, end pieces or guiding pieces having diameters bigger than the rod diameter. Conventional rock drilling rods are most often manufactured from hollow rods, and shank adapters are most often manufactured from solid rods. For threaded rods, at least one thread is often machined in a bumped up (forged) end having a diameter bigger than the rod diameter, while the shank adapters often are machined from rounds.
Instead of forging up the dimension or using extensive machining it has been more and more common to employ a friction weld to interconnect pieces having big differences in diameter. However, due to the friction-generated heat, drill equipment manufactured from conventional drill steel develops weak "soft" zones on both immediate sides (i.e., heat-affected regions) of the weld. These zones have lower hardness than the parts of the rod that are unaffected by the heat and are thus the weakest parts of the rod. To avoid the presence of soft zones in the drill rod, it is necessary to alternatively carburize, normalize or harden this type of rod after friction welding.
Another way is to compensate for the weaker strength in the soft zones by bumping up the rod end before friction welding.
During top hammer drilling under ground, e.g. production drilling or drifter drilling, a one-rod drilling method is used wherein flushing water is conducted through the rod to force the cuttings from the bored hole. Earlier these types of drilling methods often were performed via hand held machines equipped with integrals, i.e. a tool where a cemented carbide insert is brazed directly on a bumped-up end of the rod. Nowadays it is mostly highly mechanized drilling where all the machinery is carried on drill rigs and one rig often has two machines running on the same rig. With the mechanized method it has been possible to increase the effects of the machines. Therefore the rods are subjected to higher forces during both collaring and drilling. In tools for mechanized drilling, the rods are threaded and a separate bit is mounted on the rod. That means that one rod can be used much longer than an integral (one-piece) rod since the rod, instead of being discarded when the bit is worn out, can be furnished with a new bit. Water flushing results in a risk for corrosion fatigue, however, especially since the water, for example in mines, often is acidic and therefore extra corrosive.
The part of this type of rod most susceptible to corrosion fatigue is the threaded end and the clearance portion disposed between the full rod section and the thread.